Radar level gauging in tanks containing e.g. liquid, gas or granulate products has been used during several decades and a few different technical solutions are in use. For signal generation and signal processing the method called FMCW (frequency modulated continuous wave) has been extensively used especially where the measuring accuracy is a crucial specification point. In most FMCW-system a transmit signal having substantially equal amplitude and a linearly varying frequency is emitted into the tank, and is reflected by the surface. The reflected signal is received and mixed with the emitted signal, to form an intermediate frequency signal, where the intermediate frequency (IF) corresponds to the distance from the transmitter/receiver to the point of reflection (typically the surface of the product in the tank). Other types of frequency modulation instead of a linear sweep are also possible.
FMCW-systems for level gauging typically use frequencies like 6, 10 and 25 GHz and a frequency bandwidth of 1 GHz or (mainly for the 25 GHz-system) up to 2-3 GHz. Systems using around 78 GHz are also known. The internal transmission path from electronics to the antenna is kept clean with low reflections and especially the tank seal may be a bit difficult as it must also stand high pressures, various chemicals etc.
The measuring performance can be characterized in several ways but at least the following four are used on a regular basis:
Accuracy relates to how exact and reliable the measurement is. When the frequency is generated by a frequency synthesizer very high accuracy and long-term stability can be obtained. The accuracy approval is often referred to as “custody transfer approval” and (depending on the country) may be a mandatory legal requirements for certain uses. Typically the required accuracy in such cases is specified to around 1 mm. Note that in a large oil tank 1 mm level difference may correspond to a value of thousands of US dollar.
Distance resolution, or the ability to discriminate between two possible close radar echoes in the tank, is 150-200 mm or slightly more for most radar level gauges.
Updating rate is the time to next measurement. Level gauging in storage tanks is generally not very demanding (the level may change just a few cm per minute or less) and 1 measurement per second is a common acceptable minimum figure. Some applications may require much faster response by the gauging system.
Sensitivity is a measure of how weak echoes can be detected and processed by the signal processing. The maximum two-way path loss is a common way to measure sensitivity, and an FMCW-system should typically not have more than 90-100 dB two-way path loss to perform a measurement. Path loss variation between different cases can be significant.
In principle, the distance resolution capability is basically limited by the radar frequency bandwidth, but for an FMCW-system the limitation is also coupled to the signal processing and there is a trade-off to accuracy with other properties like update rate. An FMCW system needs some filtering operations (windowing of the spectrum) which are typically done digitally. The type and width of the filter window can be optimized for different purposes so the correlation between RF-bandwidth and resolution is weaker than for a pulsed system but still exists, and the bandwidth should therefore normally be at least 1 GHz.
The fact that the distance resolution is normally two orders of dignity larger in terms of distance than the required accuracy (100 mm compared to 1 mm) implies that the radar system, especially an FMCW system, is sensitive for external and internal radar reflections. If the transmission path for example contains two reflections (such as an antenna opening and the not perfectly matched connector to the antenna) there will be a smaller signal which has passed the distance between these two reflexes two extra times. If for instance both reflections have a mismatch of voltage standing wave ratio (VSWR) of 2 (reflection ˜−10 dB each) a deviation of ±10 mm may occur at worst case internal distance (within the distance resolution). If the reflections are internal and very stable this may not be a problem (careful calibration can be ensured) but for larger reflections and reflection in a variable tank environment a desired mm-accuracy is obviously in danger if the undesired and initially 10 mm deviation may change from time to time.
In most FMCW-systems a reasonable microwave match of all microwave components (i.e. VSWR<1.2-1.5) has proven to give a feasible practical solution. In very accurate FMCW-systems, however, points of strong mismatch are known to be sources of problem. Reflections from obstacles in the tank give similar influence and are eliminated by using a narrow beam antenna, and FMCW based systems typically use an antenna creating a pencil beam radiation pattern directed perpendicular down towards the surface.
To further increase accuracy, FMCW-based systems are sometimes used with a wave guiding structure in the form of a 50-100 mm pipe of stainless steel extending from the antenna horn through the entire tank. Most typically a 2 inch or 4 inch pipe is used for a 10 GHz system. Such “still-pipes” are a very efficient way to avoid possible disturbing echoes in the tank. Obviously the cost is increased by including a still-pipe, but the pipe enables undisturbed measurement when there are many disturbing objects around the pipe and when the surface sometimes has very low reflection due to a turbulent surface. The pipe, the feeding of it and possibly a valve to disconnect the parts above the tank roof (to enable change of parts) are made with a very good electric match to ensure accuracy.